It has long been known and customary to produce paints and other coatings for decorative and/or protective purposes on the most diverse types of substrates from aqueous dispersions of synthetic resins. For this purpose, very finely divided aqueous dispersions of thermoplastic or elastomeric saturated synthetic resins are applied to the object to be coated and the water present in the dispersion is removed by evaporation, soaking up by the support or both at the same time, whereby the synthetic resins particles now coming into close contact can form a continuous film. For this film-forming, a certain plasticity of the synthetic resin is required.
This plasticity can be characterized either by the hardness of the synthetic resin material or by the film-forming temperature (minimum filming temperature M.F.T.), which means the minimum temperature required for forming a continuous, homogeneous film, of the separate synthetic resin particles. This temperature can be determined, e.g., according to the method of Th. Protzman and G. Brown, Journal of Applied Polymer Science 4, 81 (1960.) A further characteristic value is the glass transition temperature T.sub.g, that is the temperature above which the polymer molecules attain a certain free mobility. It approximately corresponds to the temperature at which the polymer softens (softening point) and can be determined, in addition to other methods, by means of the thermomechanical penetration method. According to this method, T.sub.g is derived from the penetration rate of a calibrated probe tip under constant load into the coating film heated at a linear rate of 5.degree.C per minute. The M.F.T. in a first approximation is a linear function of the T.sub.g.
Synthetic resin dispersions which are to form a continuous film (M.F.T. &lt; room temperature) necessarily are to consist of very soft, plastic material and therefore also yield soft, not very strong coating of low glass transition temperature. If the dispersion is composed of a harder synthetic resin (high M.F.T. and high T.sub.g,) the dispersion applied to the substrate must be heated to a higher temperature in order to soften it above the M.F.T. and thus make possible the forming of a film. The result after cooling off is then a film of adequate continuity which is still hard enough.
But in many cases, the temperatures required for this are too high for practical application, in particular when the substrates are sensitive to heat. Due to the great advantages of dispersions as pain and coating media, respectively, (high content of solids -- about 50 percent and high molecular weight of the synthetic resin -- from about 10.sup.5 to about 10.sup.7, at still low viscosity of the total dispersion, which makes for an easy and well reproducible application to the substrate; good suitability for storage; low combustion hazard; no escaping of solvent vapors through the freshly applied coating layer, etc.) it is desirable, however, to make possible the application of dispersions in these cases also. The film-forming temperature M.F.T. can be influenced at constant hardness (constant T.sub.g) of the dispersed synthetic resin by other parameters also. The diameter of the dispersed particles, nature and amount of the surfactants employed and the type of substrate exert only a minor influence; but it is possible to substantially reduce the M.F.T. by the addition of so-called plasticizers (coalescing agents.) These agents have a dissolving or swelling effect on the synthetic resin and at the same time reduce M.F.T. as well as T.sub.g. A multitude of organic solvents for this purpose has been suggested in pertinent literature. But plastification by the addition of solvents cannot be considered a satisfactory solution of this problem, as this can only be done at the expense of several of the essential advantages in employing aqueous dispersions. The solvents previously added must be removed again after film forming in order to obtain a strong film in the end, this requires either a very long period of volatilization in air (during which the film is still soft and tacky) or further thermal treatment. In addition, the weight loss during this volatilization brings about flaws in the uniform film structure, financial losses and the forming of undesirable solvent vapors.
An attempt was made, for instance, to apply coatings to a substrate by simultaneously spraying onto it a synthetic resin dispersed in an organic carrier liquid (i.e. organosol) and a solvent, optionally on monomer basis, and to harden them there. The disadvantages of this process consist in the fact that on the one hand, technologically complicated devices are required for the simultaneous spraying of organosol and solvent and application by means of rollers or brushes is impossible and, on the other hand, fluctuations in the quantity ratios of the two components bringing about irregularities in the properties of the finished coating cannot be prevented. Moreover, there is only little time available for the required swelling of the synthetic resin particles by means of the solvent.
Further specified is a process for the production of coatings according to which an unsaturated synthetic resin is mixed with a monomer and a suspension prepared from these two components is applied to a metal base by means of an electric field and there polymerized. The range of application of this process is limited to specific types of synthetic resins with a certain number of double bonds and acid groups in the polymer molecule, which makes for a correspondingly costly and complicated production process. Moreover, the substrates are limited to electrically conductive media and even in these, technologically elaborate devices are required for the application of the coatings.
Furthermore, it is known to produce articles comprising a metallic substrate coated by applying a mixture of an aqueous dispersion of a saturated acrylic resin with a diethylenically unsaturated monomer and hardening the film obtained from that mixture by thermocatalytic means. Obviously, this process cannot be used for the production of articles comprising heat-sensitive substrates.